Electronic device having external terminals with lead-free metal thin film formed on the surface thereof

ABSTRACT

A resin sealed IC has a plurality of external terminals. A metal thin film made of a Sn-Bi alloy is formed in direct contact with the surface of a base member of each external terminal. A Bi content in the Sn-Bi alloy layer is within a range of 0.5 to 6.0 wt %. Further, the Sn-Bi alloy layer has a single-layer plating structure, and the film thickness is within a range of 10 to 25 MIC.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an electronic device in which alead-free (hereinafter referred to as “Pb-free”) metal thin filmcontaining tin (hereinafter referred to as “Sn”) as a main component isformed on the surfaces of external terminals.

[0003] 2. Description of the Prior Art

[0004] Electronic apparatuses which are used in a wide variety of fieldsare assembled of various electronic devices such as a semiconductorintegrated circuit (hereinafter referred to as “IC”), a transistor, acapacitor, a resistor and an inductor. To assemble such electronicapparatuses, a circuit board on which a circuit pattern formed by aelectrical conducting layer is printed in advance is used, and aplurality of electronic devices are mounted on the circuit board. Morespecifically, the external terminals of the electronic device areelectrically connected and physically attached to a portion of thecircuit pattern with a low-melting solder. To secure reliability ofconnection between the electronic device and the circuit board, in ametal thin film, which is made generally of a Sn-Pb alloy is formed inadvance on the surface of the external terminals of the electronicdevice by a surface treatment method such as electroplating.(Hereinafter, “reliability of connection between the electronic deviceand the circuit board” is referred to as “bonding reliability”.)

[0005] However, Pb presents a danger to public health and causesenvironmental pollution when a used electronic apparatus is discarded.Thus, a use of the Sn-Pb alloy is not desirable from the viewpoint ofenvironmental protection. Under such circumstances, a materialcomprising a Sn-based alloy containing no Pb, which is the so-calledPb-free Sn-based alloy, is required to be used as the low-meltingsolder. It is also required that a metal thin film comprising thePb-free Sn-based alloy is formed on the surface of a lead base materialof an electronic device by plating.

[0006] When the metal thin film comprising the Pb-free Sn-based alloy isformed on the surface of the base member by plating, it is importantthat the metal thin film, which does not impair the wettability of thelow-melting solder and can secure bonding reliability, is formedregardless of which metal is chosen as an additive metal to be added toSn. As an example, an electronic device in which a Sn-bismuth(hereinafter referred to as “Sn-Bi”) alloy is plated on the surface of abase member as a metal thin film is widely known. As in the case of Pbin the above Sn-Pb alloy, Bi is a metal which forms a low-melting alloytogether with Sn and lowers the melting point of the alloy. Applying theSn-Bi alloy to the metal thin film of external terminals is disclosed inJapanese Patent Laid-Open Publication Nos. 2000-174191, 2001-257303, Hei11-330340, 2001-53211, 2002-151838, 2002-141456 and Hei 11-251503, andU.S. Pat. Nos. 6,195,248, 6,392,293 and 6,395,583, and U.S. patentapplication Publication No. US2002/0019077.

[0007] When a Sn-Bi alloy is used as the above Pb-free Sn-based alloyand a metal thin film comprising the Sn-Bi alloy is formed on thesurface of a base member by plating, the growth of fine metal whiskersis liable to occur on the surface of the external terminal when anelectronic device is subjected to an acceleration test such as atemperature cycling test after production of the electronic device, ascompared with a case where the metal thin film is formed by use of anSn-Pb alloy. Further, it is concerned that these whiskers may makeshort-circuit between the adjacent external terminals. Suchshort-circuit is more likely to take place in a semiconductor devicesuch as an IC in which a plurality of external terminals are derivedfrom the periphery of a package body at minute intervals. In addition,since the Sn-Bi alloy has poor ductility, bending cracks (hereinaftersimply referred to as “cracks”) are liable to occur in the Sn-Bi alloylayer when the external terminals are bent upon, e.g., implementation ofthe electronic device. Therefore, suppression of the occurrences ofwhiskers growth and cracks formation is significantly important when ametal thin film comprising a Pb-free Sn-based alloy is formed on thesurface of a base member of an electronic device by plating.

[0008] This discussion for suppressing the occurrences of whiskersgrowth and cracks formation in a Pb-free metal thin film, formed on thesurface of a base member are made in Japanese Patent Laid-OpenPublication Nos. 2000-174191, 2001-257303 and Hei 11-330340.

[0009] Japanese Patent Laid-Open Publication No. 2000-174191 discloses asemiconductor device having a lead of which a cross sectional structureis shown in FIG. 12. The lead of the semiconductor device is formed byplating a lower layer 102 which comprises an Sn-Bi alloy having a Bicontent of 0.7 wt % (weight %), an intermediate layer 103 whichcomprises an Sn-Bi alloy having a Bi content of 0.7 to 2.3 wt % and anupper layer 104 which comprises an Sn-Bi alloy having a Bi content of2.3 wt % on a surface of a base member 101. The three Sn-Bi alloy layershaving different Bi contents have such a concentration gradient that thecontents of the alloy components increase in a plating film thicknessdirection.

[0010] Further, Japanese Patent Laid-Open Publication No. 2001-257303discloses a lead material for electronic devices which has a crosssectional structure as shown in FIG. 13. On a surface of a base member111 of the lead material for electronic devices, a plating layer 112which comprises an Sn-Cu alloy having a Cu content of 0.4 to 5 wt % anda film thickness of 1 to 15 micrometers (hereinafter referred to as“MIC”) is formed.

[0011] Further, Japanese Patent Laid-Open Publication No. Hei 11-330340discloses a semiconductor device having a lead of which a crosssectional structure is shown in FIG. 14. The lead of the semiconductordevice is formed by plating a lower layer 122 which comprises an Sn-Bialloy having a Bi content of 0 to 1 wt % and a film thickness of 1 to 14MIC and an upper layer 123 which comprises an Sn-Bi alloy having a Bicontent of 1 to 10 wt % and a film thickness of 1 to 12 MIC on a surfaceof a base member 121.

[0012] It is, however, recognized by present inventor that the aboveprior arts have the following problems.

[0013] The conventional semiconductor devices use the Pb-free Sn-basedalloys as the metal thin films to be formed on the surfaces of the basemembers, and the metal thin films are formed by multilayer plating so asto suppress the occurrences of whiskers growth and cracks formation, sothat a plating step becomes complicated and costs of a plating processis driven up.

[0014] Further, along with the multilayer plating structures, strictcontrol of the contents of the metals to be added to Sn becomesdifficult.

[0015] In addition, since the contents of the metals to be added to Snand the plating film thicknesses are not adjusted to right values inaccordance with the kinds of the metals to be added to Sn, it isdifficult to sufficiently suppress the occurrences of whiskers growthand cracks formation.

[0016] For example, it is known that when the above Sn-Bi alloy isplated as a metal thin film to be formed on the surface of a base memberof an external terminal, reliability of connection between the externalterminal and a circuit board after the electronic device is mounted onthe circuit board by soldering is significantly influenced by thecontent of Bi in the metal thin film. Therefore, it is an importantpoint for guaranteeing product quality to strictly control the Bicontent in the metal thin film. For that purpose, the content of Bi inthe metal thin film is estimated with high accuracy in a non-destructivemanner by a method such as a fluorescent X-ray analysis.

SUMMARY OF THE INVENTION

[0017] According to an aspect-of the present invention, there isprovided an electronic device that includes a plurality of externalterminals each having a base member and a metal thin film formed indirect contact with a surface of the base member, the metal thin filmbeing made of an alloy of tin and bismuth and the bismuth beingcontained in the alloy so as to satisfy any one of the followingconditional expressions;

20≦Xm≦25 and 0.5≦Cam≦4.5,   (a)

15≦Xm≦20 and 0.7≦Cam≦4.5,   (b)

10<Xm≦15 and 4.5≦Cam≦6.0,   (c)

[0018] wherein Xm indicating the thickness (MIC) of the metal thin filmand Cam indicating wt % of the bismuth in the metal thin film.

[0019] According to another aspect of the present invention, there isprovided an electronic device that includes a plurality of externalterminals each having a base member and a metal thin film formed indirect contact with a surface of the base member, the metal thin filmbeing made of an alloy of tin and bismuth and the bismuth beingcontained in the alloy so as to satisfy any one of the followingconditional expressions;

10<Xm≦25, 0.5≦Cam≦6.0 and −8Cam+46<Xm≦−8Cam+54,   (a)

10<Xm≦25, 0.5≦Cam≦6.0 and −5Cam+25≦Xm≦−8Cam+46,   (b)

10<Xm≦25, 0.5≦Cam≦6.0 and −5Cam+15≦Xm<−5Cam+25,   (c)

[0020] wherein Xm indicating the thickness (MIC) of the metal thin filmand Cam indicating wt % of the bismuth in the metal thin film.

[0021] In the electronic device thus constructed according to thepresent invention, the occurrences of whiskers growth and cracksformation on the external terminals thereof can be sufficientlysuppressed under the simple structure of the metal thin film formed indirect contact with the surface of the base member, and the Bi contentadded to Sn can be controlled strictly.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The above and other objects, advantages and features of thepresent invention will be more apparent from the following descriptiontaken in conjunction with the accompanying drawings, in which:

[0023]FIG. 1 is an oblique perspective view of an electronic devicewhich is a first embodiment of the present invention;

[0024]FIG. 2A is a plan view of the electronic device of FIG. 1;

[0025]FIG. 2B is a cross sectional view along the line I-I of FIGS. 1and 2A;

[0026]FIG. 3 is a schematic diagram showing a cross sectional structureof a portion of an external terminal of the electronic device of FIG. 1;

[0027]FIG. 4 is a diagram showing an example of ranges of preferredcombinations of the Bi content and plating film thickness of a Sn-Bialloy layer formed on the external terminal of the electronic device ofFIG. 1;

[0028]FIG. 5 is a diagram showing another example of ranges of preferredcombinations of the Bi content and plating film thickness of the Sn-Bialloy layer formed on the external terminal of the electronic device ofFIG. 1;

[0029]FIG. 6 is a diagram showing the results of evaluation of whiskersin the relationship between the Bi content and plating film thickness ofthe Sn-Bi alloy layer formed on the external terminal of the electronicdevice of FIG. 1;

[0030]FIG. 7 is a diagram showing the occurrence of cracks in therelationship between the Bi content and plating film thickness of theSn-Bi alloy layer formed on the external terminal of the electronicdevice of FIG. 1;

[0031]FIG. 8 is a diagram showing the evaluation results of variationsin measurement of plating composition in the relationship between the Bicontent and plating film thickness of the Sn-Bi alloy layer formed onthe external terminal of the electronic device of FIG. 1;

[0032]FIG. 9 is a schematic diagram showing a cross sectional structureof a portion of an external terminal of an electronic device which is asecond embodiment of the present invention;

[0033]FIG. 10 is a schematic diagram showing a cross sectional structureof a portion of an external terminal of an electronic device which is athird embodiment of the present invention;

[0034]FIG. 11A is an oblique perspective view of an example of anelectronic device (lead-insertion-type transistor) to which the presentinvention is applied;

[0035]FIG. 11B is an oblique perspective view of an example of anelectronic device (surface-mounting-type transistor) to which thepresent invention is applied;

[0036]FIG. 11C is an oblique perspective view of an example of anelectronic device (electrolytic capacitor) to which the presentinvention is applied;

[0037]FIG. 12 is a schematic diagram showing a cross sectional structureof a portion of lead of a conventional semiconductor device;

[0038]FIG. 13 is a schematic diagram showing a cross sectional structureof a portion of a conventional lead material for electronic devices; and

[0039]FIG. 14 is a schematic diagram showing a cross sectional structureof a portion of lead of a conventional semiconductor device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040] The invention will be now described herein with reference toillustrative embodiments. Those skilled in the art will recognized thatmany alternative embodiments can be accomplished using the teachings ofthe present invention and that the invention is not limited to theembodiments illustrated for explanatory purpose. In the followingdescription, a resin sealed IC will be used as an example of theelectronic device.

First Embodiment

[0041] Referring now to FIG. 1, FIG. 2A and FIG. 2B, a resin seated IC10 has such a constitution that a plurality of external terminals 2 isderived from both sides of a package body 1 which is encapsulated with aresin. A base member 2 a of the external terminal 2 is composed of aconductive material, e.g., an iron-nickel alloy (hereinafter referred toas “Fe-Ni alloy”). A metal thin film 3 made of a Sn-Bi alloy is formedin direct contact with the surface of the base member 2 a by a surfacetreatment method such as electroplating.

[0042] In the package body 1, an IC chip 4 is bonded on a die pad 5 withan adhesive or the like which is not shown. Pad electrodes 6 which areformed on the surface of the IC chip 4 are electrically connected tocorresponding internal lead portions 2 b by, for example, bonding wires7. The base member 2 a of the external terminal 2 extends to acorresponding internal lead portion 2 b. These are integrally formed.

[0043] In the present embodiment, the metal thin film 3 is made of theSn-Bi alloy layer 3 a. The content of the Bi in the Sn-Bi alloy layer 3a (hereinafter referred to as “the Bi content”) is within a range of 0.5to 6.0 wt %. The thickness of the Sn-Bi alloy layer 3 a (hereinafterreferred to as “the film thickness”) is controlled to fall within arange of over 10 to 25 MIC. Since the Sn-Bi alloy layer 3 a is formed indirect contact with the surface of the base member 2 a byelectroplating, costs of a plating process cannot be driven up. Further,the film thickness is so controlled as to be within a right range inaccordance with the Bi content.

[0044]FIG. 4 is a diagram showing an example of regions of preferredcombinations of the Bi content and the film thickness when the Sn-Bialloy layer 3 a is formed in direct contact with the surface of the basemember 2 a. In FIG. 4, the diagram is shown on an orthogonal coordinatesystem, wherein X axis stands for Cam indicating wt % of Bi in the Sn-Bialloy layer 3 a, Y axis stands for Xm indicating the thickness (MIC) ofthe Sn-Bi alloy layer 3 a and preferred combinations of Cam and Xm areexpressed as corresponding coordinate regions.

[0045] Referring to FIG. 4, the content of Bi is within a range of 0.5to 6.0 wt %, and the film thickness is controlled so as to fall within aright range in accordance with the Bi content. For example, when thewettability of a low-melting Pb-free solder is important, the Bi contentis required about 4 wt % and above, and by adjusting the film thicknessto be within the range of over 10 to 15 MIC, the superior physicalproperties can be attained on the whole. On the other hand, when it isrequired to keep the Bi content less than 1 wt %, the film thickness iscontrolled to be within a range of 20 to 25 MIC. Hereinafter, firstembodiment will be described in detail.

[0046] A region P1 shown in FIG. 4 indicates that the film thickness ispreferably controlled to be within the range of 20 to 25 MIC when the Bicontent is within a range of 0.5 to 4.5 wt %. A region P2 indicates thatthe film thickness is preferably controlled to be within the range of 15to 20 MIC when the Bi content is within a range of 0.7 to 4.5 wt %. Aregion P3 indicates that the film thickness is preferably controlled tobe within the range of over 10 to 15 MIC when the Bi content is within arange of 4.5 to 6.0 wt %. By controlling the film thickness so as tofall within the right ranges in accordance with the Bi content asdescribed above, the occurrences of whiskers growth and cracks formationin the Sn-Bi alloy layer 3 a can be sufficiently suppressed withoutimpairing the wettability of the low-melting solder. Further, since theaccuracy of estimation of the Bi content can be also improved, exactmanagement of the Bi content becomes possible, and reliability ofbonding between the external terminal and a mounting board can besecured.

[0047] Then, referring to FIG. 5, a region Q1 shown in FIG. 5 is aregion wherein a combination of Cam indicating wt % of the Bi of theSn-Bi alloy layer 3 a and Xm indicating the thickness (MIC) of the Sn-Bialloy layer 3 a satisfies all the following expressions:

10<Xm≦25, 0.5≦Cam≦6 and −5Cam+25≦Xm≦−8Cam+46.

[0048] When a combination of Cam and Xm is within the region Q1, theSn-Bi alloy layer 3 a is free from the occurrence of cracks formation inbending the external terminal 2 and the occurrence of whiskers growth.

[0049] A region Q2 shown in FIG. 5 is a region wherein a combination ofCam and Xm satisfies all the following expressions:

10<Xm≦25, 0.5≦Cam≦6 and −5Cam+15≦Xm<−5Cam+25.

[0050] When a combination of Cam and Xm is within the region Q2, theSn-Bi alloy layer 3 a has no cracks formation occurring in bending theexternal terminal 2, although the occurrence of very short whiskersgrowth, which cannot make a short-circuit between the adjacentterminals, may still be barely observed in the Sn-Bi alloy layer 3 a.

[0051] A region Q3 shown in FIG. 5 is a region wherein a combination ofCam and Xm satisfies all the following expressions:

10<Xm≦25, 0.5≦Cam≦6 and −8Cam+46<Xm≦−8Cam+54.

[0052] When a combination of Cam and Xm is within the region Q3, theSn-Bi alloy layer 3 a is free from the occurrence of whiskers growth,although fine cracks, which are not large enough to reach the basemember, may be observed in bending the external terminal 2.

[0053] Therefore, when neither the cracks formation in bending theexternal terminal 2 nor the whiskers growth is acceptable, a combinationof Cam and Xm must be adjusted so as to fall within the region Q1.

[0054] Further, when the occurrence of fine cracks which are not largeenough to reach the base member is permitted, although the whiskersgrowth is not acceptable, a combination of Cam and Xm should be adjustedto fall within the region Q1 or Q2. More specifically, a combination ofCam and Xm should be adjusted so as to satisfy all the followingexpressions:

10<Xm≦25, 0.5≦Cam≦6, −5Cam+15≦Xm<−8Cam+46.

[0055] Meanwhile, when a few, very short whiskers growth, which willnever make a short-circuit between the adjacent external terminals, isacceptable although the cracks formation in bending the externalterminal 2 is not acceptable, a combination of Cam and Xm should beadjusted to fall within the region Q1 or Q3. More specifically, acombination of Cam and Xm should be adjusted so as to satisfy all thefollowing expressions:

10<Xm≦25, 0.5≦Cam≦6 and −5Cam+25<Xm≦−8Cam+54.

[0056] In addition, when a few, very short whiskers growth and the finecracks, which are not large enough to reach the base member, occurred inbending the external terminal 2 are acceptable, a combination of Cam andXm should be adjusted to fall within any one of the regions Q1, Q2 andQ3. More specifically, a combination of Cam and Xm should be adjusted soas to satisfy all the following expressions:

10<Xm≦25, 0.5≦Cam≦6 and −5Cam+15≦Xm≦−8Cam+54.

[0057] In this case, a permissible range of a combination of Cam and Xmis significantly expanded, thereby improving productivity.

[0058] Next, evaluation results from which the regions of FIGS. 4 and 5have been derived will be described.

[0059] (1) About Wettability of Solder

[0060] In the Sn-Bi alloy layer 3 a, the wettability of a solderimproves as the Bi content increases. However, even if the Bi contentbecomes zero, in other words, Sn constitutes 100%, the wettability ofthe solder is still acceptable from a practical standpoint. Meanwhile,it has been confirmed that when the film thickness is about 3 MIC andbelow, it becomes difficult to keep up sufficient wettability due to theoccurrence of pinholes and the like therein, so that the film thicknessis desirably adjusted to about 5 MIC and above, more preferably over 10MIC. Thus, no deterioration in the wettability of the solder occurs byadjusting the Bi content to falling within the range of 0.5 to 6.0 wt %and the film thickness to falling within the range of over 10 to 25 MIC,as shown in FIG. 4 or 5.

[0061] (2) About Whiskers

[0062] The whiskers growth was evaluated as follows: preparing aplurality of samples having different combinations of the Bi content andthe film thickness, wherein the Sn-Bi alloy layer 3 a of each of aplurality of samples is formed by plating, keeping the samples in anatmosphere where whiskers growth often occurs for a predetermined time,then observing the whiskers.

[0063]FIG. 6 shows an orthogonal coordinate system similar to that ofFIG. 4 or 5 which shows the observation results for the whiskers growth.A symbol N denotes a coordinate point corresponding to a combination ofCam and Xm of the sample in which no whiskers growth was observed. Asymbol A denotes a coordinate point corresponding to a combination ofCam and Xm a sample in which fine nodules were observed. A symbol Bdenotes a coordinate point corresponding to a combination of Cam and Xmof a sample in which very short whiskers growth which cannot makeshort-circuit between the adjacent external terminals was barelyobserved. A symbol C denotes a coordinate point corresponding to acombination of Cam and Xm of a sample in which a number of whiskersgrowth were observed. A symbol D denotes a coordinate pointcorresponding to a combination of Cam and Xm of a sample in whichwhiskers having the potential of making short-circuit between theadjacent external terminals are observed.

[0064] The following can be taught by referring to FIG. 6.

[0065] (1) With a Bi content of lower than about 3 wt %, the whiskersgrowth is hardly observed by adjusting the film thickness to about 10MIC and above.

[0066] (2) With a Bi content of about 5 wt % and above, the whiskersgrowth can be inhibited even if the plating film is thin.

[0067] (3) When a combination of Cam and Xm is in a region above a firstline represented by a formula Xm=−5Cam +15, i.e., a region whichsatisfies −5Cam+15≦Xm, only very short whiskers growth, which cannotmake short-circuit between the adjacent external terminals, is barelyobserved, and multiple whiskers growth are not observed.

[0068] (4) When a combination of Cam and Xm is in a region above asecond line represented by a formula Xm=−5Cam+25, i.e., a region whichsatisfies −5Cam+25≦Xm, no whiskers growth is observed.

[0069] Thus, the following is understood with respect to the whiskersgrowth. That is, when the Sn-Bi alloy layer 3 a is used as the metalthin film 3, the whiskers growth can be suppressed sufficiently, evenwith a plating structure in direct contact with the surface of the basemember 2 a, by adjusting a combination of Cam and Xm to fall within anyone of the regions P1, P2 and P3 shown in FIG. 4 or any one of theregions Q1, Q2 and Q3 shown in FIG. 5.

[0070] (3) About Cracks

[0071] The cracks formation was evaluated by using, as a sample to beevaluated, a resin-sealed semiconductor device (not shown) comprising aTQFP (Thin Quad Flat Package) having 80 pins. An external terminal ofthe not-shown semiconductor device is formed by electroplating a Sn-Bialloy as a metal thin film 3 on the surface of a base member comprisinga Fe-Ni alloy. A cross section of the external terminal has the samestructure as that of FIG. 2. Further, a plurality of samples havingdifferent combinations of the Bi content, Cam in wt %, and plating filmthickness, Xm in MIC, of the Sn-Bi alloy layer 3 a were prepared so asto evaluate the cracks formation in the Sn-Bi alloy layers 3 a at leadbended “R” portions (shown in FIG. 2) of the external terminals of thesamples after the external terminals have gone through lead bendingprocedure. FIG. 7 is a diagram showing an orthogonal coordinate systemsimilar to FIG. 4 or 5 which shows the cracks formation in the Sn-Bialloy layers 3 a in bending the external terminal 2. In FIG. 7, a symbol∘ (white circle) denotes a coordinate point corresponding to acombination of Cam and Xm of a sample in which no cracks formation wasobserved. A symbol Δ (trigon) denotes a coordinate point correspondingto a combination of Cam and Xm of a sample in which the fine cracksformation being not large enough to reach the base member was observed.A symbol × (multiple mark) denotes a coordinate point corresponding to acombination of Cam and Xm of a sample in which the cracks formationcoming down to the base member was observed.

[0072] The following can be taught by referring to FIG. 7.

[0073] (1) Taken as a whole, the cracks formation is reduced as the filmthickness and the Bi content decrease.

[0074] (2) With the film thickness of about 10 MIC and below, only finecracks formation can be observed even if the Bi content is about 5 wt %.

[0075] (3) With the film thickness of about 20 MIC, the cracks formationcan be observed even if the Bi content is about 4 wt %.

[0076] (4) When a combination of Cam and Xm is in a region below a thirdline represented by a formula Xm=−8Cam +54, i.e., a region whichsatisfies Xm≦−8Cam+54, only the fine cracks formation which is not largeenough to reach the base member is observed, and no cracks formationcoming down to the base member was observed.

[0077] (5) When a combination of Cam and Xm is in a region below afourth line represented by a formula Xm=−8Cam +46, i.e., a region whichsatisfies Xm≦−8Cam+46, no cracks formation including micro cracks isobserved at all.

[0078] Thus, the following is understood with respect to the cracksformation. That is, it is preferred to keep the Bi content at about 3 wt% or lower and the film thickness at about 10 MIC or smaller, and it isundesirable to have a Bi content of about 4 wt % or higher and a filmthickness of about 20 MIC or larger.

[0079] Consequently, when the Sn-Bi alloy layer 3 a is used as the metalthin film 3, the cracks formation can be suppressed sufficiently byadjusting a coordinate point corresponding to a combination of Cam andXm to fall within any one of the regions P1, P2 and P3 shown in FIG. 4or any one of the regions Q1, Q2 and Q3 shown in FIG. 5.

[0080] (4) About Accuracy of Measurement of Bi Content

[0081]FIG. 8 is a diagram illustrating, on an orthogonal coordinatesystem similar to that shown in FIG. 4 or 5, an example of evaluatingvariations in the results of measurements of the Bi contents. The Bicontent was measured by use of a fluorescent X-ray analysis using aproportional counter as a detector. In FIG. 8, numeral allocated to eachof the coordinate points represents coefficient of, variation of theobserved values for the content of Bi in Sn-Bi alloy layer 3 a which hasCam and Xm corresponding to each of the coordinate points. (Coefficientof variation is obtained by dividing standard deviation of the resultsof fixed-point repetitive measurements by their average value.) Thesmaller the value is, the smaller the variation in the measured valuesof the Bi content.

[0082] The following can be taught by referring to FIG. 8.

[0083] (1) In a region where the film thickness is over about 10 MIC,the Bi content can be measured with practical accuracy.

[0084] (2) In a region where the film thickness is as thin as about 5MIC and the Bi content is less than about 1 wt %, variations becomeextremely large, and measurement accuracy deteriorates.

[0085] Thus, to take strict control of composition within a region wherethe Bi content is less than about 1 wt %, the film thickness must beabout 10 MIC and above.

[0086] Consequently, when the Sn-Bi alloy layer 3 a is used as the metalthin film 3, variations in measurement of the Bi content can becontrolled and measurement accuracy can be improved by adjusting acoordinate point corresponding to a combination of the Bi content andthe film thickness to fall within any one of the regions P1, P2 and P3shown in FIG. 4 or the regions Q1, Q2 and Q3 shown in FIG. 5.

[0087] (5) About Bonding Reliability

[0088] When an electronic device having an Sn-Bi alloy layer 3 a platedon the surfaces of external terminals is implemented on a circuit boardor the like by an Sn-Bi alloy solder, the total Bi content in theimplemented/bonded system (plating+soldering) determines bondingreliability for the assembled circuit board. Thus, to secure bondingreliability, the Bi content in the Sn-Bi alloy layer 3 a must be socontrolled as to fall within a range corresponding to theimplemented/bonded system. With the film thickness of about 10 MIC andabove, sufficient measurement accuracy can be maintained and bondingreliability can be secured even if the Bi content is less than 1%.

[0089] The foregoing regions shown in FIG. 4 were established based onthe results of evaluations and considerations of the wettability,occurrence of whiskers growth, occurrence of cracks formation, variationin measurement of the Bi content and bonding reliability. Further, theregions shown in FIG. 5 were also set based on the results ofevaluations and considerations of the wettability, occurrence ofwhiskers growth, occurrence of cracks formation, variation inmeasurement of the Bi content and bonding reliability, particularly inconsideration of the first and second lines of FIG. 6 and the third andfourth lines of FIG. 7.

[0090] Thus, according to the resin sealed IC 10 of the firstembodiment, the Sn-Bi alloy layer 3 a, which is formed in direct contactwith the surface of the base member 2 a of the external terminal 2through plating, is formed as the metal thin film 3. Further, the Bicontent is within a range of 0.5 to 6.0 wt %. In addition, the filmthickness is controlled within a range of over 10 to 25 MIC inaccordance with the Bi content so that a combination of the Bi contentand the film thickness would fall within any one of the regions shown inFIG. 4 or 5. Hence, the occurrences of whiskers growth and cracksformation can be suppressed as an application of the electronic devicewithout impairing the wettability of the low-melting solder. Further,the accuracy of measurement of the Bi content can also be improved, sostrict control of the Bi content becomes possible, and the bondingreliability of the external terminals can be secured.

Second Embodiment

[0091]FIG. 9 is a schematic diagram showing a cross sectional structureof a portion of an electronic device which is a second embodiment of thepresent invention. The constitution of the electronic device of thesecond embodiment is significantly different from the constitution ofthe above first embodiment in that the electronic device of the secondembodiment uses a Sn-silver (hereinafter referred to as Sn-Ag) alloylayer 3 b in place of Sn-Bi alloy layer 3 a as a metal thin film 3. Theelectronic device of the second embodiment is constituted almost in thesame manner as the first embodiment of FIGS. 1 and 2. That is, aplurality of external terminals 2 which comprise, e.g., an Fe-Ni alloyare drawn out from both sides of a package body 1 which is encapsulatedwith a resin, and inside the package body 1, an IC chip 4 is bonded on adie pad 5 by an adhesive or the like which is not shown. Further, padelectrodes 6 which are formed on the surface of the IC chip 4 areelectrically connected to corresponding internal lead portions 2 b by,for example, bonding wires 7. The base member 2 a of the externalterminal 2 extends to a corresponding internal lead portion 2 b. Theseare monolithically formed. Further, as a metal thin film 3, the Sn-Agalloy layer 3 b is formed in direct contact with the surface of the basemember 2 a by a surface treatment method such as electroplating.

[0092] In the second embodiment, as shown in FIG. 9, the metal thin film3 is made of a Sn-Ag alloy resulting from addition of Ag to Sn and has asimple structure. Further, the content of Ag in the Sn-Ag alloy layer 3b. is 2.0 to 4.0 wt % and the film thickness of the Sn-Ag alloy layer 3b is 15 to 25 MIC. Although specific data will be omitted, regionscorresponding to preferred combinations of the Ag content and the filmthickness of the Sn-Ag alloy layer 3 b as the metal thin film 3 in thesecond embodiment were set by making the same studies as those in thecase of the Sn-Bi alloy layer 3 a in the first embodiment.

[0093] The second embodiment can further improve the resistance to thecrack formation, as compared with the Sn-Bi alloy layer 3 a of the firstembodiment, by adjusting the film thickness of the Sn-Ag alloy layer 3 bso as to fall within a right range in accordance with the Ag content inthe Sn-Ag alloy layer 3 b. However, wettability and the resistance tothe whisker growth are slightly degraded as compared with the Sn-Bialloy layer 3 a of the first embodiment. Further, by choosing apreferred combination of higher Ag content and more thick plating film,external terminal's bonding reliability and the accuracy of measurementof the Ag content can be improved almost in the same manner as in thefirst embodiment.

[0094] Thus, same effects as those described in the first embodiment canbe obtained with the constitution of the second embodiment.

Third Embodiment

[0095] The constitution of the electronic device of the third embodimentis significantly different from the constitution of the above firstembodiment in that the electronic device of the third embodiment uses aSn-zinc (hereinafter referred to as Sn-Zn) alloy layer 3 c in place ofSn-Bi alloy layer 3 a as a metal thin film 3. The electronic device ofthe third embodiment is constituted almost in the same manner as thefirst embodiment of FIGS. 1 and 2. In the third embodiment, the metalthin film 3 is made of a Sn-Zn alloy resulting from addition of Zn to Snand has a simple structure.

[0096] Referring now to FIG. 10, a Sn-Zn alloy layer 3 c is formed indirect contact with the surface of the base member 2 a by a surfacetreatment such as electroplating. Further, the content of Zn in theSn-Zn alloy layer 3 c is 4.0 to 9.0 wt % and the film thickness of theSn-Zn alloy layer 3 c is adjusted to fall within a range of 15 to 30MIC.

[0097] In the third embodiment, by adjusting the film thickness of theSn-Zn alloy layer 3 c so as to fall within a right range in accordancewith the Zn content in the Sn-Zn alloy layer 3 c, the occurrences ofwhiskers growth and cracks formation can be sufficiently suppressedwithout impairing wettability, as in the case of the first embodiment.Further, external terminal's bonding reliability and the accuracy ofmeasurement of the Zn content can also be improved.

[0098] Thus, same effects as those described in the first embodiment canbe obtained with the constitution of the third embodiment.

[0099] It is apparent that the present invention is not limited to theabove embodiments, but may be modified and changed without departingfrom the spirit and scope of the invention. For example, in theforegoing embodiments, the lead-shaped external terminal having themetal thin film formed thereon has been described. However, the type ofthe external terminal is not limited to the lead type, externalterminals having any type such as a bump or film can be used as long asthey can function as external terminals. Further, in the foregoingembodiments, an IC has been used as an electronic device, and the casewhere the present invention has been applied to the external terminalsof the IC has been described. However, in addition to the IC, thepresent invention is applicable to the external terminals of otherelectronic devices such as the external terminals 21 of alead-insertion-type transistor 11 as shown in FIG. 11A, the externalterminals 22 of a surface-mounting-type transistor 12 as shown in FIG.11B, and the external terminals 23 of an electrolytic capacitor 13 asshown in FIG. 11C.

[0100] Further, as a surface treatment method for forming the metal thinfilm made of the Sn-based alloy, such as the Sn-Bi alloy, the Sn-Agalloy and the Sn-Zn alloy, on the surface of the base member of theexternal terminal of the electronic device, electroplating has beendescribed as an example thereof. However, other plating methods such aselectroless plating or a combination of electrolytic plating and theelectroless plating can also be applied. Further, the Fe-Ni alloy hasbeen used as an example of the base member on which the metal thin filmof the Sn-based alloy is formed according to the present invention. But,the base member is not limited to the Fe-Ni alloy, and a Fe-Ni-basedalloy which further contains other metal components may also be used. Inaddition, the base member is not limited to the Fe-Ni-based alloy, andmaterials such as Cu, a Cu-based alloy which contains Cu as a maincomponent and Fe may also be used.

What is claimed is:
 1. An electronic device comprising: a plurality ofexternal terminals each having a base member and a metal thin filmformed in direct contact with a surface of the base member, the metalthin film being made of an alloy of tin and bismuth and the bismuthbeing contained in the alloy so as to satisfy any one of the followingconditional expressions; 20≦Xm≦25 and 0.5≦Cam≦4.5,   (a) 15≦Xm≦20 and0.7≦Cam≦4.5,   (b) 10<Xm≦15 and 4.5≦Cam≦6.0,   (c) wherein Xm indicatingthe thickness (MIC) of the metal thin film and Cam indicating wt % ofthe bismuth in the metal thin film.
 2. An electronic device comprising:a plurality of external terminals each having a base member and a metalthin film formed in direct contact with a surface of the base member,the metal thin film being made of an alloy of tin and bismuth and thebismuth being contained in the alloy so as to satisfy any one of thefollowing conditional expressions; 10<Xm≦25, 0.5≦Cam≦6.0 and−8Cam+46<Xm≦−8Cam+54,   (a) 10<Xm≦25, 0.5≦Cam≦6.0 and−5Cam+25≦Xm≦−8Cam+46,   (b) 10<Xm≦25, 0.5≦Cam≦6.0 and−5Cam+15≦Xm<−5Cam+25,   (c) wherein Xm indicating the thickness (MIC) ofthe metal thin film and Cam indicating wt % of the bismuth in the metalthin film.
 3. An electronic device comprising: a plurality of externalterminals each having a base member and a metal thin film formed indirect contact with a surface of the base member, the metal thin filmbeing made of an alloy of tin and silver and the silver being containedin the alloy so as to satisfy the following conditional expression;15≦Xm≦25 and 2.0≦Cam≦4.0, wherein Xm indicating the thickness. (MIC) ofthe metal thin film and Cam indicating wt % of the silver in the metalthin film.
 4. An electronic device comprising: a plurality of externalterminals each having a base member and a metal thin film formed indirect contact with a surface of the base member, the metal thin filmbeing made of an alloy of tin and zinc and the zinc being contained inthe alloy so as to satisfy the following conditional expression;15≦Xm≦30 and 4.0≦Cam≦9.0, wherein Xm indicating the thickness (MIC) ofthe metal thin film and Cam indicating wt % of the silver in the metalthin film.
 5. The electronic device as claimed in claim 1, wherein themetal thin film is formed by plating.
 6. The electronic device asclaimed in claim 1, wherein the base member is composed of a conductivematerial.
 7. The electronic device as claimed in claim 6, wherein theconductive material comprises a metal selected among the group includingan iron-nickel alloy, an iron-nickel-based alloy, copper, a copper-basedalloy and iron.
 8. The electronic device as claimed in claim 2, whereinthe metal thin film is formed by plating.
 9. The electronic device asclaimed in claim 2, wherein the base member is composed of a conductivematerial.
 10. The electronic device as claimed in claim 9, wherein theconductive material comprises a metal selected among the group includingan iron-nickel alloy, an iron-nickel-based alloy, copper, a copper-basedalloy and iron.